The present invention relates to a writable and rewritable optical recording medium, a tracking method for the medium and an optical recording/reproducing apparatus for recording and reproducing information on/from the optical recording medium.
Recently, writable and rewritable optical discs, which are used as storage means for personal computers and as package media for music and video information, have been developed to achieve higher recording density.
Each optical disc has a writable and rewritable area and a pre-pit area previously formed as a pit thereon for storing information that must not be erased.
Portions other than the pre-pit area are writable areas in which xe2x80x9cgrooves (track)xe2x80x9d and xe2x80x9clands (not track)xe2x80x9d are formed.
A typical structure of a conventional optical disc is shown in FIGS. 1 and 2. FIG. 1 is a plan view and FIG. 2 is a perspective, partly in cross-section, of the optical disc.
In FIG. 2, G denotes a groove, L denotes a land and PP denotes a preformed pit (hereinafter referred to as pre-pit). A laser light beam 3 is collected by an objective lens 2 and illuminates the recording surface of the disc through a substrate 1. The grooves G are nearer to the objective lens 2 than the lands L. The lands L, grooves G and pre-pits PP are coated with a recording layer (not shown) made of magneto-optical material or phase-change material or photosensitive dye material. In the shown case, record marks M are recorded in grooves. This is because marks recorded in grooves can achieve higher quality of reproduced signal than marks recorded on lands between the grooves.
The following is an example of a method for optimally selecting a depth of the groove G and a depth of the pre-pit PP to be formed on an optical disk.
The example represents the experimental results made on optical discs, which have the same track pitch (inter-groove distance) of 0.74 microns but differ from each other by their groove depth Dg and pre-pit depth Dp, by using an optical system composed of a laser emitting a light beam of a wavelength xcex=650 nm and an objective lens NA0.6. The groove G and the pre-pit PP are of 0.35 microns in width. The recording layer made of phase-change material InAgSbTe was applied. The recording and reproducing were carried out by rotating each disc at a linear velocity of 3.5 m per second.
FIG. 3 shows amplitudes of reproduced signals obtained, respectively, from marks recorded in grooves of different depths Dg and from pre-pits having different depths Dp.
More specifically, a number of optical discs having different groove depths Dg and different pre-pit depths Dp were subjected to measurements of amplitudes of reproduced signals obtained from 0.3 micron long marks recorded in grooves G and amplitudes of reproduced signals obtained from 0.3 micron long pre-pit.
The measurement results shown in FIG. 3 indicate that the marks recorded in shallower grooves have larger amplitude of reproduced signals, i.e., better S/N ratios. This means that it is preferable to decrease the depth Dg of groves G to improve the S/N ratio of reproduced signals of marks thereof. This offers a great advantage in particular for discs of higher recording density.
On the other hand, tracking of a light beam focused on groove G is needed to achieve orderly recording information in the form of marks M in the grooves G and precisely reproducing the information. For this reason, the depth of the grooves G must be decided in view of an amplitude characteristic of a signal reproduced from the mark and an amplitude characteristic of tracking signal (i.e., a push-pull signal) obtained on the basis of an average intensity distribution of light components reflected in a direction perpendicular to a direction of the grooves G.
FIG. 4 is a graph showing the dependence of amplitudes of push-pull signals obtained from grooves G and from pre-pits PP upon groove depth Dg and pre-pit depth Dp respectively.
In the graph, xcex denotes a wavelength of a light beam and n denotes a refractive index of a substrate of an optical recording medium.
As seen from the graph, the maximal amplitude of the push-pull signal can be obtained when the groove depth Dg or the pre-pit depth Dp is equal to xcex/(8n). This means that the grooves G having the depth Dg of xcex/(8n) are desired for obtaining push-pull signals being large enough to achieve the precise tracking. However, in view of the amplitude of the reproduced signal obtained from a mark, it is preferable to select the groove depth Dg being smaller than xcex/(8n). For example, a depth value indicated by A in FIGS. 3 and 4 is about 20 nm at xcex=650 nm and n=1.5 and it is preferable to obtain a large push-pull signal as well as an improved S/N ratio of the reproduced signal of the mark.
On the other hand, it is found from the relationship between the pre-pit depths Dp and amplitudes of reproduced pre-pit signals (FIG. 3) that the amplitude of the reproduced prepit signal can achieve a maximal value at the pre-pit depth Dp of xcex/(4n) and decreases as the pre-pit depth Dp decreases. Hence, a depth value B (about 100 nm in FIGS. 3 and 4) may be selected as the pre-pit depth Dp. This selection, however, may be accompanied by decreasing the amplitude of the push-pull signal at the pre-pit depth of about xcex/(4n) as shown in FIG. 4. Namely, it is difficult to increase both the amplitude of the reproduced pre-pit signal and the amplitude of the pre-pit push-pull signal.
In other words, it is difficult to use the push-pull signals for tracking in the pre-pit areas. Therefore, the use of differential phase detection (DPD) method, which is different from the push-pull method by its detection principle, is desirable for tracking in the pre-pit area. This method obtains information necessary for tracking by detecting a change of a refraction pattern of light beam illuminating the surface of the optical recording medium (optical disk) and reflected therefrom, or by detecting the differential phase of the refraction pattern change.
FIG. 5 shows the relationship between the depths Dp of pre-pits and the amplitudes of DPD signals obtained from the pre-pits.
As shown in FIG. 5, the DPD method is suited to tracking in the pre-pit areas since it can obtain a large-amplitude tracking signal from pre-pits having the depth Dp of about xcex/(4n), at which the tracking signal obtained by the pushpull method has a very small amplitude.
Returning to FIG. 1, problems involved in an optical recording medium having grooves and pre-pits will be discussed. FIG. 1 is a plan view of an optical recording medium constructed of grooves G having the depth Dg of A and pre-pits PP having the depth Dp of B.
The combination of the selected grooves G having the depth Dg=A with the selected pre-pits PP having the depth Dp=B requires switching from the tracking method for the groove areas to the tracking method for the pre-pit areas and vice versa. Namely, the push-pull method is applied to a groove area while the DPD method is applied to a pre-pit area. Otherwise, effective tracking signals cannot be obtained.
The switching of tracking methods is desirable to be carried out within a very nallow area designated for this purpose. The reason is as follows: If the switching operation timing was shifted out of the tracking mode switching area, mismatched tracking, e.g., the DPD tracking would be conducted in a shallow groove area or the push-pull tracking would be conducted in a deep pre-pit area until the switching operation is accomplished. Consequently, the correct tracking control could not be realized.
The tracking mode switching area exists between the groove area and the pre-pit area. This area has a very short length of several microns. When the optical recording medium is rotated at a linear velocity of 1 to several meters per second, light beam passes this area in only a several microseconds. In other words, the tracking mode switching must be done for several microseconds.
Although only the tracking mode switching can be conducted in a moment by means of an electronic switch, it is still needed in practice to previously recognize which area, groove area or pre-pit area, is currently illuminated by the light beam. This operation normally requires several hundred microseconds. In other words, the tracking cannot be correctly controlled and hence is unstable until the tracking mode switching including the recognition of the currently illuminated area is completed. This causes misalignment of the mark M out of the center of the groove G when recording or a reproduction error from the deterioration of the reproduced signal when reproducing information. Furthermore, if any external disturbance is applied to the device, the beam spot is shifted from the target track, resulting, in the worst case, in recording or reproducing no information. Namely, there arises a problem of decreasing the stability and the reliability of the recording and reproducing apparatus.
Even if the beam spot position is fixed until the tracking mode switching is completed, no tracking control is conducted for that duration and hence the above problem remains unsolved.
The pre-pits normally carry non-erasable information such as address information, disc-related information, video information, music information etc. The grooves are normally vacant allowing the user to write desirable information thereon by using the recording/reproducing apparatus. In the combination of pre-pits with grooves, the user can normally write information in a groove area following a pre-pit containing an address. In this instance, there is still a problem that a groove area writable by the user is reduced by providing pre-pits of address information. This may be solved by an art disclosed in Japanese Laid-open Patent Publication No. 11-73686, whereby a land pre-pit (LPP) is provided in a land area between grooves and is read with a push-pull signal.
In such a disc consisting of pairs of a groove, a pre-pit and a land pre-pit, a portion, e.g., an internal circumferential area is a ROM area of non-erasable information such as disc-related information, video information and music information and another portion following the ROM area is a writable area (i.e., a groove area provided with LPPs) whereon the user can write any desirable information. The disc of this type can provide the user with ROM information prepared by the video and music program providers and allows the user to write information by own preferences.
As pointed out in the Japanese Laid-open patent Publication No. 11-73686, the LPP information must be obtained as a cross talk over recorded information and hence the obtainable signal quality is unstable. As seen in FIG. 6B, in the disc having land pre-pits (LPP) provided on a deep pre-pit area (depth B) and a shallow groove area (depth A), the LLPs neighboring to the deep pre-pit in comparison with the LPPs disposed in the groove area are inferior in its signal quality due to the cross-talk of the ROM information. Address information cannot be correctly read from that portion. Therefore, the disc of this type is practically constructed with the LPPs only in the groove area and with no LPP in the deep pre-pit area as shown in FIG. 6A to prevent inferior in its signal quality of ROM information.
However, the recording/reproducing apparatus, which usually reads address information from the LPPs, cannot recognize the address information until a light beam enters the groove area of the disc constructed as shown in FIGS. 6A and 6B. Since the recording device obtains address information and then conducts recording operation, it cannot record data on the top of the groove area of the disc. Namely, there may be an area lacking a recorded mark. This means a decrease in amount of writable information and the presence of a (gap) area with no RF-signal between a ROM area and a user area on the recoded disc. For a reproducing device that conducts tracking based on the differential phase detection (DPD) method, the gap area means absence of a tracking signal, whereby the beam may be out of the track in the area.
The present invention was made to solve the above-mentioned problems.
Accordingly, an object of the present invention is to provide an optical recording medium capable of obtaining large reproduced signals from recorded marks and pre-pits and large tracking signals in groove areas and pre-pit areas, and providing time allowance enough to switch over tracing mode between groove area and pre-pit area; a tracking method for recording and reproducing on and from the optical recording medium; and the optical recording and reproducing apparatus for using the optical recording medium. In other words, the present invention is intended to provide an optical recording medium that has the high data-reliability and high tracking accuracy and does not allow tracking error due to timing error of tracking-mode switching; a tracking method; and an optical recording and reproducing apparatus for recording and reproducing information on and from the optical recording medium.
Another object of the present invention is to provide an optical disc that has both a ROM area and a user area and has addresses recorded on LPPs on an optical disc, on which a recording device can perform recording with no gap between the ROM area and the user area to form continuous tracks composing the ROM area and the user area.
Another object of the present invention is to provide an optical recording medium comprising a substrate whereon tracks each comprising of at least grooves and pits deeper than or equal to the grooves are formed and areas of shallow pits are each provided between an area of the deep pits and an area of the grooves.
Another object of the present invention is to provide an optical recording medium, characterized in that a depth Dg of the groove, a depth Dsp of the shallow pit and a depth Ddp of the deep pit satisfy the following conditions:
Dg less than xcex/(8n),
xcex/(8n) less than Ddp less than xcex/(4n)
and
Dgxe2x89xa6Dsp less than Ddp,
where xcex is a wavelength of a light beam illuminating the optical recording medium and n is a refractive index of the substrate.
Another object of the present invention is to provide an optical recording medium, characterized in that at least one of the grooves or pit trains in an area of the pit is wobblingly formed.
Another object of the present invention is to provide an optical recording medium, characterized in that pre-pits is provided between the grooves.
Another object of the present invention is to provide a tracking method for optical recording media, characterized in that tracking is conducted based on a phase difference of reflected light in the areas of the deep pits and based on an average intensity distribution difference of reflected light in the areas of the groove and tracking is switched from the tracking based on the phase difference of the reflected light to the tracking based on the average intensity distribution difference of the reflected light and vice versa in the areas of the shallow pits.
Another object of the present invention is to provide a tracking method, characterized in that timing of switching the tracking is obtainable from address information recorded on the optical recording medium.
Another object of the present invention is to provide a tracking method, characterized in that timing of switching the tracking is obtainable from RF-signal amplitude information or RF-signal error information or wobble amplitude information.
Another object of the present invention is to provide an optical recording and reproducing apparatus for recording and reproducing information on and from any one of the optical recording media, which conducts tracking based on a phase difference of reflected light in the areas of the deep pits and tracking based on an average intensity distribution difference of reflected light in the areas of the groove and switches tracking mode from the tracking based on the phase difference of the reflected light to the tracking based on the average intensity distribution difference of the reflected light and vice versa in the areas of the shallow pits.
Another object of the present invention is to provide an optical recording and reproducing apparatus, characterized in that timing of switching the tracking mode is obtained from address information recorded on the optical recording medium.
Another object of the present invention is to provide an optical recording and reproducing apparatus, characterized in that timing of switching the tracking mode is obtained from RF-signal amplitude information or RF-signal error information or wobble amplitude information.
Another object of the present invention is to provide an optical recording medium comprising a substrate whereon tracks each consisting of at least an area of deep pits, an area of shallow groove and an area of shallow pits are formed and pre-pits are provided between neighboring tracks in the areas of the shallow groove and shallow pit.
Another object of the present invention is to provide an optical recording medium, characterized in that a part of the each area of the deep pits is replaced by an area of the shallow pits.
Another object of the present invention is to provide an optical recording and reproducing apparatus for recording and reproducing information on and from the optical recording media, which conducts receiving address information or timing information from pre-pits provided between the tracks and starts recording information on a head of a groove formed following the area of the shallow pits.